This invention relates to the fabrication of electrode plates for lead-acid electrochemical cells, and more particularly to a processing aid for improving the characteristics of slurry-type lead battery pastes.
Lead-acid electrode plates are typically prepared by mixing oxides of lead or blends thereof with a dilute solution of sulfuric acid. A blend of litharge (PbO) and red lead (Pb.sub.3 O.sub.4) may be used for the positive plate and leady oxide, consisting of mainly litharge with added free lead particles, is commonly used for the negative plate. Various bulking agents may be employed, expanders for the negative plate to prevent densification and loss of capacity during cycling, and various other additives. In the conventional process the addition of sulfuric acid to the blend of lead oxides and other additives results in an exothermic reaction. The paste readily becomes very "stiff", and dough-like in consistency. Typically this stiff, sulfated paste is applied with an open-feed gear pump discharging onto a moving conductive substrate of lead grid or expanded mesh, for instance. Generally a doctor blade is used to scrap off the excess paste. It is generally impossible with this process to apply a constant volumetric flow rate of paste to a given length of grid and, hence, the amount of electrochemically active lead material applied to a given area of grid may vary from plate to plate.
An aqueous-based, slurry-type paste considerably more fluid than the conventional stiff, dough-like pastes was developed for sealed lead-acid cells and batteries of the type disclosed in U.S. Pat. No. 3,862,861 (McClelland et al.). By using an aqueous vehicle or lesser concentration of sulfuric acid the resultant viscosity of these slurries is considerably less than the traditional pastes, allow pumping and metering in a continuous fashion, as taught in U.S. Pat. No. 3,814,628 (Larkin). These slurry-type pastes of lower viscosity are compatible with soft, pure lead grids of the type preferred in the aforementioned U.S. Pat. No. 3,862,861, and may be continuously extrusion coated onto a moving substrate.
Continuous extrusion coating processes of this type result in application of substantially a precise amount of paste per length of grid resulting in uniformity in discharge capacity of the resultant plate. Processes and apparatus for extrusion coating slurry-type pastes of this kind are disclosed in U.S. Pat. No. 3,894,886 (Pankow et al.) and U.S. Pat. No. 4,050,482 (Ching et al.). Slurry pastes of the type contemplated hereunder, whether formulated with water only, or lightly sulfated with the appropriate levels of dilute sulfuric acid, produce thixotropic slurries of a suspension of fine solid particles in an aqueous vehicle.
These finely divided particles of solids of lead and lead oxides are generally macromolecular in size and naturally tend to settle. To keep the lead and lead oxide solids suspended in the liquid vehicle, nonionic thickeners, notably hydroxypropyl cellulose (e.g. Klucel supplied by Hercules Inc.) has been used as a processing aid to increase slurry fluidity by reducing its viscosity, and to retard settling. However, nonionic thickeners of this type present a number of drawbacks. The hydroxypropyl cellulose thickener must be premixed with water, rather than being added directly to the paste mixer as a powder. The hydroxypropyl cellulose solution foams. Furthermore, the hydroxypropyl cellulose thickener in solution undergoes acid catalyzed hydrolysis and the solution is slightly acidic (with sulfuric acid) to minimize transformation of massicot (yellow, rhombic lead oxide) to litharge (red, tetragonal lead oxide). Hydrolysis of the hydroxypropyl cellulose thickener leads to solutions of lower viscosity (lower molecular weight), complicating process control.
Furthermore, cells made with hydroxypropyl cellulose thickener as a processing aid are retained within the cell, and may degrade cell performance since at least the carbon black portion of the expander e.g. including barium sulfate and lining sulfonate, does not appear to be uniformly dispersed in the paste slurry. Additionally, the hydroxypropyl cellulose thickener does not permit achievement of a sufficiently high solids content in the paste as desired, resulting in producing negative plates in particular which are lower in bulk density than desired. The high discharge rate performance of cells made with hydroxypropyl cellulose thickener also appears to be impaired.
It is an object of this invention to produce electrode plates for lead-acid cells by applying slurry-type suspensions onto a suitable substrate using a processing aid which permits extrusion of a dense but fluid (workable) slurry onto a suitable substrate.
It is a further object to prepare such an electrode using a processing aid which can be added as a solid directly to the electrode powder mix, which is stable during cell manufacture e.g. resisting hydrolysis, and which disperses the negative plate expander and retards settling of the solid contents of the slurry during processing.
It is a further object to provide the aforementioned processing aid and not degrade cell performance and, in fact, enhance high discharge rate performance.